For many years, V-belts have functioned as strong and reliable drive elements. They are used, for instance, to power accessory units, and are used in many capacities in motor vehicles. The geometry of the V-belt must match the groove of the pulley on which used.
The width of the groove which characterizes the grooved profile is designated as the guiding width of the pulley groove and usually lies within the height of the effective zone of a drawn-up V-belt inside the pulley groove. It should normally coincide within the range of tolerance values with the effective width of the groove.
On the other hand, the reference width of the pulley groove is measured at the radially outer end of the straight laterally opposite faces of the groove as viewed in a section through the axis of the pulley. Furthermore, in the case of a multiple groove V-belt pulley, the groove spacing is a decisive dimension. In such a V-belt pulley, the groove spacing is measured from groove center to groove center of adjacent grooves of the same size.
The groove spacing may also be the minimum spacing between grooves of the same size on two individual belt pulleys in side-by-side relationship but constituting power transmitting components in two different belt drives.
In order to transfer ever greater loads, a multitude of belts have been developed for which the tension cord band, which is embedded within the height of the effective load transferring zone in order to increase the power transferring performance of a given size belt pulley, has increasingly been moved radially toward the outer surface of the belt. By such construction, the tension cord band which is embedded within the effective load transferring zone can be wider and, therefore, more efficient.
In the case of a conventional sheathed V-belt, the tension cord band is located between the rubber center and the outer diameter, and the pulley grooves are protected by an abrasion resistant mesh sheathing. In the case of an open-faced belt, the mesh sheathing is not used and therefore the belt, especially within the effective zone, can be designed to be wider. An open-face belt has an anisotropic composition. Because of the use of fibers which are aligned crosswise with respect to the run of the belt, the requirements for bending strength, on the one hand, and for a high degree of abrasion resistance along the faces as well as a high degree of cross-sectional rigidity, on the other hand, are met. Washing out of the exposed and partially cut tension fibers during use is prevented by a preparation which guarantees a secure adhesion within the embedding mixture.
The open-face belt can be cut to an exact shape and contour which leads to considerable improvement of the operational noise emission and makes it possible to individually adapt it as a pulley belt. Basically, the open-face V-belt offers the advantages of higher bending and cross-sectional strength, a lower rate of elongation, a lower rate of slipping, and higher power transfer performance. In the case of high speed belts, extremely small belts and high performance requirements, the open-face belt is significantly superior to the sheathed V-belt.
The effective power transmitting zone of the tension cord band of an open-face V-belt is located a significant distance above the reference width of a belt pulley whereby, with belt drives of equal design dimensions, a markedly higher level of power transfer can be attained because of the wider effective power transmitting zone. With a V-belt effective zone of this type, which is located radially outside the belt pulley, the edges of the pulley groove (transition from the groove faces to the cylindrical periphery surface of the belt pulley) come into contact with the belt faces. In order to avoid damage to the belt faces caused by the edges and/or, as the case may be, the transition area, the edges are customarily chamfered or rounded.
A break in the edges caused by chamfering creates new edges which can damage the belt. Rounded edges at the transition area may of necessity have a very small radius of curvature because too large a transition radius increases considerably the outer diameter of the belt pulley. Increases in a pulley diameter may make it difficult or impossible to install a V-belt because in some belt drives an adjustable tie rod for changing pulley spacing has very little adjustment, thus requiring the belt to be stretched, when, in fact, such belts can be lengthened only to a minor extent. Furthermore, a transition with a large radius of curvature also increases the overall axial dimension of the belt pulley, whereby the width of the belt drive is increased. In practice, therefore, a small radius of curvature is predominantly used at the outer end of the transition.
The forming of small radii rounded transitions of the hereinbefore mentioned type can result in trenches in the faces of the belt groove which results in considerable damage to the V-belt. Experience has shown that, after several hours operation, mechanical detachments occur in faces of the belt, which causes the belt to sink deeper into the pulley groove. When this happens, the stiff tension cord band part of the belt, which is crossways to the run of the belt, comes into power transmitting contact with the groove faces, and the belt is supported on them, whereby because of the high degree of cross-sectional resistance of the belt, the normal friction contact area of the belt, which is located beneath the tension cord band, does not contact groove faces. As a result, the tension cord band must now take on a disproportionately large part of the cross-sectional force needed to cause the V-belt to transmit the desired power. The narrow range of the effective power transmitting zone is considerably overloaded mechanically because of this. Because of the limited contact area between the belt faces and the groove faces, the belt begins to slip, wears out considerably in the process, becomes overheated and malfunctions after a short period. Furthermore, there is the danger that the tension cord band which is located in the area of the reference width will be damaged directly by the imperfect groove contour so that the power transmitting structure of the belt is destroyed.
Whenever the belt drive exhibits misalignment between the driving pulley and the driven belt pulley, belt damage caused by the imperfect contour in the transition area will result in failure of the belt after only a few hours' operation. Heretofore, in order to guarantee sufficient useful life for belt drives used for high power transmissions, the belt as well as the belt pulleys needed to be manufactured as precisely as possible and fitted to each other and assembled within a narrow range of tolerances.